Multi-Forms Binding data

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C#

WPF

My project have two WPF Forms: Form1 and Form2. In Form1 I have 1 button to call Form2, textBox1, textBox2, textBox3, textBox4, Form2 has only one textBox and a Save button. So when I click button, it show Form2. In textBox I make a template text like:

"blablabla %txt1% blablabla %txt2% blabla %txt3% blabla"

I click Save button to Save it. When return Form1, textBox4 will display content in template text in which %txt1%, %txt2%,%txt3% will change depend on textBox1, textBox2, textBox3. I intend to use MultiBinding to bind content in textBox1,2,3 into textBox4, it like that:

<TextBox Name="textBox4">
 <TextBox.Text>
  <MultiBinding StringFormat = "blablabla {0} blablabla {1} blabla {2} blabla"
   <Binding ElementName = "textBox1" Path="Text"/>
   <Binding ElementName = "textBox2" Path="Text"/>
   <Binding ElementName = "textBox3" Path="Text"/>
  </MultiBinding>
 </TextBox.Text>
</TextBox>

And my problem: how to get

"blablabla {0} blablabla {1} blabla {2} blabla"

from textBox in Form2 and put it to StringFormat? I'm mean I can't use Bind data from textBox in Form2 to MultiBinding.